Thin film resistors



July 17, 1962 Filed Aug. 25, 1958 D. A. SKOQG THIN FILM RESISTORS 2Sheets-Sheet 1 INVENTOR. 00061145 19. 5/1006 July 17, 1962 D. A. SKOOGTHIN FILM RESISTORS 2 Sheets-Sheet 2 Filed Aug. 25, 1958 J m P .Qsam MHRQ EM'PJED was, my:

United States atent 3,044,903, THIN FILM RESISTORS Douglas A. Skoog,Stanford, Calif., assignor to Philco Corporation, Philadelphia, Pa., acorporation of Pennsylvania Filed Aug. 25, 1958, Ser. N 0. 756,884 1Claim. (Cl. 117-229) This invention relates generally to the art offabricating electrically conductive films, and more particularly to aunique and simplified method for manufacturing electrically conductivefilms of high operational stability and uniformity and to the article ofmanufacture pro- 'duced by such a method. I

While of broader applicability. this invention has particular utility inthe manufacture of thin-film, electrically conductive coatings on glass,ceramics and other glasslike materials and it is in this particularfield of application that the invention is illustrated and described.

It is well known in the resistor art to produce thin-film resistorsby'immersing a heated blank of glass, glazed ceramic or the like in asolution of an appropriate metal salt or by exposing the blank to thevapors or atomized spray of such a solution. These techniques, however,while resulting in the formation of a strongly adherent layer ofmetallic oxide on the supporting surface produce a resistive filmlacking electrical stability and one which is acutely sensitive to smalldeviations from prescribed manufacturing regimen as well as being highlysusceptible to post-manufacturing contamination. Moreover, conventionalthin-film resistors exhibit a marked variance in resistivity with changein ambient temperature, and while there are processes currentlyavailable for producing resistive films of increased stabilityandconstant resistivity, such processes are not easily controllable orproductive of a uniform end product.

Accordingly it is a principal object of this invention to provide areadily workable, simplified process for insuring optimum uniformity ofmass produced, thin-film resistors, particularly thin-film resistors ofthe type having constant resistivity over an extended temperature range.

A further object of the invention is to provide a simplified method formanufacturing electrically conductive films having high intrinsicelectrical and chemical stability and further to provide a novel processfor producing resistive films which are insensitive topost-manufacturing contamination.

A still further object of this invention is to provide a' non-criticalfilm forming process for mass producing stable, uniform, thin-filmresistors.

These and other objects within contemplation will be more readilyunderstood by reference to the following detailed description anddrawings, in which:

FIGURE 1 shows one arrangement for fabricating thinfilm resistors inaccordance with the present invention;

FIGURE 2 is an enlarged showing of a preferred mode of forming anelectrically conductive film;

FIGURE 3 is a graph showing the effect of arsenic on the electricalresistance of tin-oxide films;

FIGURE 4 is a graph showing the effect of varying concentrations ofarsenic on the change in resistivity of tin-arsenic films over atemperature span of 600 degrees; and

FIGURE 5is a graph illustrating the effect of arsenic doping on filmstability and reproducibility.

Briefly described, this invention in its preferred aspect, relates to aunique technique for impregnating or doping a vapor-deposited,electrically conductive film, which comprises adding to a liquidsolution of a film-forming tin salt a soluble compound of arsenic havinga range of vapor pressures permitting the controllable vaporization of aprescribed amount of the arsenic solute along with the solvent filmingvapor. This composite vapor is then transported into impingement with aheated blank of insulating support material and forms a thinelectrically conductive film on the surface portions of the insulatingsubstrate. This technique provides optimum and uniform dispersion of thearsenic additive throughout the filming vapor thereby insuring a film ofhomogeneous composition. Thin-film resistors made by this methodaccordingly exhibit unprecedented stability and uniformity of electricparameters. The entire process, once the desired composition of theelectrically conductive film has been fixed is essentially a onevariable system, the concentration of the constituents in the vaporbeing a function of their vapor pressures which in turn is a function oftemperature.

The fundamental concept underlying hte invention is the discovery thatarsenic can be used as a film stabilizing element and that when arsenicis found in a form having a vapor pressure conducive to the release ofthe desired concentration of arsenic when placed in solution with theprincipal film forming salt it provides a unique method for producingthin-film resistors which method is readily controllable and productiveof a more uniform end product, and further, results in minimizing thecomplexity of vapor filming equipment conventionally required.

One arrangement of apparatus for practicing the method steps of thisinvention is that shown in FIGURE 1. The preferred, but notexclusive'technique, is to provide a temperature-controlled solution orpool of an appropriate filming salt, such as liquid anhydrous stannicchloride into which a measured amount of a soluble arsenic salt, as forexample arsenic trichloride has been added. The arsenic solute is sochosen that its range of vapor pressures permits it to coexist with itssolvent in the vapor phase at some selected and readily attainabletemperature of operation. The concentration of the arsenic in thefilming vapor is a function, once having fixed the amount of arsenic insolution, of its vapor pressure which for all practical purposes isexclusively a function of temperature. Accordingly, once the amount ofarsenic trichloride in the solution has been fixed, the concentration ofarsenic may be maintained at a constant level by merely controlling thetemperature of the solution during the filming process.

The amount of arsenic to be placed in solution is determined by the endresult desired, as will be discussed more fully hereinafter, theparticular arsenic compound to be used being determined in large part bythe properties of the filming salt employed insofar as the arsenic com-'pound should, for most satisfactory results, be readily miscible withthe main filming constituent. It will be. recognized, however, thatwhile the preferred method of forming the electrically conductive filmis by vapor deposition, the ordinary techniques such as sprayandimmersion coating are still available and the use of arsenic in suchapplications comes within the general purview of the invention.

Referring again to FIGURE 1 the filming reagents are convenientlycontained in a closed container or flask 10 provided with a deliverytube 11. Fumes of the reagent solution are controllably generated byimmersing the flask in a temperature controlled water bath 12. Whenusing a solution comprised of anhydrous stannic chloride and arsenictrichloride as the filming reagent, a temperature of 55 C. wasconveniently employed. Accurate control of the temperature wasmaintained by using conventional thermostatic means, not shown,interposed in a connection between the temperature sensing element orthermocouple 13 and the water-bath heating coils 14. Arsenic trichlorideis readily miscible with anhydrous stannic chloride,

the admixture forming a single homogeneous phase. To

insure a fixed and unvarying proportion of arsenic in the vaporemanating from the liquid pool the temperature of the water jacket must,as noted, be carefully controlled, this being satisfactorilyaccomplished in the embodiment illustrated through the high thermalinertia of the water and surrounding parts which serve to minimizetemperature fluctuations induced by transient variations in supplyvoltage and other extraneous causes.

Substances commonly employed as substrate or foundation material in thefabrication of thin-film resistors include materials such as refractoryglass, surface-glazed ceramic and other similar insulating media. Thepreferred mode of fabrication in order to insure maximum uniformity ofthe end product is to first clean the surface of the insulating blank onwhich the coating is to be deposited, as by immersing in hot cleaningsolution, followed by several rinses in distilled water. Beforeinitiating vapor flow it is necessary, in order to insure uniform andproper adhesion of the filming material, to preheat the blank to oventemperature. A temperature ranging from 600 C. to 650 C. was foundsatisfactory when using a refractory type glass such as the pyrex tubing16 shown in FIG- URE 1. When thermal equilibrium is attained, vapor fiowis started by opening the main air supply control valve 17 which affordscommunication with compressed air supplied via the conduit 17 throughpressure reducing valve 13. The air is passed to manifold 19 and fromthere is selectively admitted into lines 20 and 21, the quantity of airpassing through these lines being individually regulated by means ofindividual control valves 22. The rate of flow in each line is measuredby the air flow meters 23. To exercise optimum control over the moisturecontent of the air it is necessary to first dehumidify the air bypassing it through a dessicant, such as silica gel, contained in flasks24 placed in each air line. After moisture extraction, the dried airflowing through line 21 is directed over the liquid pool 15 so as toentrain in the air stream the vapors emanating from the pool and tocarry the vapors through the insulated conduit 25 into impingement withthe heated blank of Pyrex tubing 16, located, as shown in phantom inFIGURE 1, within the oven 26. It has been found that the humidity insidethe furnace during the filming process is an important determinant ofthe quality of the surface coating, best results being obtained byintroducing some moisture into the film forming vapor stream. Humiditycontrol is effectively maintained by passing a stream of dry air throughline 20 over a closed flask 27 of heated water. To produce a confluentstream of entrained moisture and filming vapors, as is preferred, theconduits 25 and 28 are brought into concentric relation, the arrangementbeing clearly shown in FIGURE 2. The exit ports are arranged to producea blending of the emerging vapors prior to their impingement on theheated blank 16 thereby insuring a uniform mixture of vapor and saltfumes in the reaction zone. The glass blank 16 is simultaneously rotatedand propelled through this vapor stream by the spindle and lead-screwmechanism 29. The glass sleeve 16 is wedged or otherwise fixedly mountedon the rotatable stainless steel quill or shaft 30 journalled onstanchions 31, the shaft being powered by the motor-pulley combination32. The entire assembly carried by the support 33 is reciprocablytranslatable into and out of the electric oven 26 by nut means 34threadedly engaging the motor driven lead-screw 35.

One illustrative set of operating parameters employed to produce aconstant resistivity (16 ohm per square) resistor, utilizing Pyrex glasstubing having an outside diameter of .810 inch, was the following. Thereaction zone 36 within the oven 26 was maintained at approximately 625C. After temperature stabilization air, at the rate of 10 cc. per minutewas passed through Water flask 27, the Water being heated to 78 C.,While 900 cc. of air per minute was passed through flask 10 containing asolution of anhydrous stannic chloride having approximately 0.4% arsenictrichloride by volume heated to 55 C. During maintenance of the aboveconditions the blank 16 was rotatingly translated through themoisturized vapor stream 37. To lay down a uniform 16 ohm per squareresistive coating in one pass of the blank through the vapor streamunder the above operating conditions required the blank to be moved at atraversing speed of 3 inches per minute and a rotative speed of rpm.

The vapors, upon emerging from the concentrically disposed vents 25 and28 mix and impinge on the heated resistor blank 16 where they decomposeand immediately react to produce a doped tin oxide coating. The purposeof the water vapor, as mentioned above, is to assist the reactionprocess, and, in effect, to controllably air condition the reactionzone.

It is not known with certainty whether the final coating is tin oxidedoped with arsenic oxide or whether the arsenic goes in as asubstitutional metallic impurity in place of the tin in the crystalstructure. It is believed, however, that the material is an oxide of tinwith metallic arsenic serving as a substantial type of dopant.

The vapors within the oven 25 which have not entered into the reactionare wihdrawn through a draft type exhaust port 38 to prevent vaporaccumulation.

To tailor a resistor to a specified value of resistance, it is merelynecessary to modify the thickness of the resistive coating, this beingmost readily accomplished by modifying the residence time of the blankwithin the filming vapors.

Numerous tests were run using arsenic as a film stabilizing additive.Some of the more important effects are graphically illustrated inFIGURES 3 through 5. The films were prepared in much the same mannerdiscussed above by exposing the heated blank to gaseous mixtures of air,stannic chloride, and arsenic trichloride. For each set of data, 5010.5ml. of stannic chloride was transferred to a flask and heated to 55 C. Afew resistors were prepared from the vapor above this solution. A smallvolume of arsenic trichloride was then introduced into the anhydrousstannic chloride from a microburet. One or more resistors were thenprepared from the vapor of this solution. The concentration of arsenictrichloride was then increased by a second addition of the reagent andmore resistors were prepared. In this way a series of resistorscontaining various arsenic concentrations was obtained.

Resistance of the samples was determined by passing a measured DC.current through the resistor and measuring the potential drop between apair of pointed contacts held firmly against the resistor at a fixeddistance apart. The precision of such measurements is believed to befrom 2 to 3% The effect of arsenic upon the electrical resistivity oftin oxide films is graphically illustrated in FIGURE 3, the arseniccontent being plotted on a logarithmic scale for convenience ofillustration. Two series of resistors were prepared in the manner aboveindicated using identical operating parameters except where specificallynoted. In one series 'a considerably heavier film was imparted to theglass by using a greater flow rate of the vapor and by increasing theoven residence time of the blank. Curve A of FIGURE 3 represents themean resistance of resistors prepared using an increased flow rate of902 cc. of air per minute at a traversing speed of 3 inches per minute.Curve B represents the mean resistance of resistors prepared usingvarying concentrations of arsenic and an intermediate flow rate of 680cc. per minute at a traversing speed of 6 inches per minute. It will benoted that as little as .003 ml. of arsenic trichloride in 50 ml. ofanhydrous stannic chloride results in a 15 to 30% decrease in theelectrical resistance of the vapor deposited film. Further increases inarsenic concentration have little effect up to about .03 ml. Beyond thispoint a rapid rise in the resistance is observed.

The presence of arsenic in the conducting films was found to have aprofound efiect upon the temperature coefficient of resistance asgraphically shown in FIG- URE 4. -In the absence of arsenic the tinoxide films undergo a 20% change in resistance throughout a change inambient temperature equal to approximately 600 C. It will be seen,however, that with the addition of even trace amounts of arsenic thepercentage change becomes smaller and decreases to Zero whenapproximately 0.2 ml. of arsenic trichloride are placed in solution withthe main filming constituent. It will be observed that further increasein the arsenic content of the filming solution results in an abruptreversal of this resistance stabilizing efi'ect producing resistorshaving increased sensitivity to temperature change. It will be furthernoted that the zone of stabilization occurs at approximately the samearsenic concentration as that at which the rapid rise in resistance isobserved in FIGURE 3.

Another condition which should be noted is that there is a change insign of the temperature coefficient at about 0.2 ml. of arsenic per 50ml. of anhydrous st-annic chloride, the resistance of the blanks belowthis point increasing with increasing temperature whereas at greaterconcentrations their resistance decreases with increasing temperature.

As pointed out earlier, arsenic impregnated films are less sensitive topost-manufacturing contamination. To demonstrate this property numerousthin-film resistors were prepared over a four day interval using afilming solution containing 0.2 ml. or arsenic trichloride per 50 ml. ofanhydrous stannic chloride. As a control, films were also prepared froma solution of unadulterated anhydrous stannic chloride. The proceduresused were, as far as possible, identical, and the samples and controlswere prepared within a few minutes of one another. The results are shownin FIGURE 5. Curve A is representative of films containing arsenic andcurve B of films containing no arsenic. With the films containingarsenic the average deviation of the mean resistance as measured using aplurality of similarly made resistors was ohms, while the filmscontaining no arsenic exhibited an average deviation of 40 ohms. Themaximum deviation was 40 ohms in one case and 170 ohms in the second.This data indicates clearly that the uncontrollable variables in theprocess have a considerably smaller effect on films containing arsenic.

The novel use of arsenic in the fabrication of thininexpensive andnon-critical technique for making films having essentially a zerotemperature coeificient of resistance.

While the invention has been described with particular reference tospecific practice and embodiments, it will be understood by thoseskilled in the art that it is susceptible to changes and modificationswithout departing from the scope thereof, as defined in the appendedclaim.

We claim:

A method of producing electric resistors of a predetermined temperaturecoeflicient adjacent zero and of a resistivity which is highly stable inthe presence of postmanufacturing contamination, said method comprisingthe steps of: providing a solution consisting of liquid anhydrous:stannic chloride and liquid anhydrous arsenic trichloride, the ratio ofarsenic trichloride to stannic chloride in said solution ranging fromapproximately .2% to approximately .4% by volume, and maintaining saidsolution at a temperature of approximately C. to produce a filming vaporcontaining tin and arsenic Whereby said filming vapor has an arseniccontent predetermined by the arsenic vapor pressure which correspondswith said temperature; evolving Water vapor; and simultaneously applyingsaid filming and water vapors to exposed surfaces of a refractory bodyheated to several hundred degrees C. to produce on said body anelectrically conductive film of tin, doped with arsenic in proportionwith said arsenic content of the filming vapor.

References Cited in the file of this patent, UNITED STATES PATENTS2,375,482 Lyle May 8, 1945 2,478,817 Gaiser Aug. 9, 1949 2,564,706Mochel Aug. 21, 1951 2,564,707 Mochel Aug. 21, 1951 2,602,032 GaiserJuly 1, 1952 2,772,190 Haayman et a1. Nov. 27, 1956 FOREIGN PATENTS702,774 Great Britain Jan. 20, 1954

